The present invention relates to a multichannel flat tube for a heat exchanger, having a plurality of parallel flow channels located side by side in the transverse direction of the tube.
A flat tube of this kind for use in a condenser of a cooling or air conditioning system is known from European patent EP 0 219 974 B1, with the parallel flow channels in the flat tube being formed by a corrugated insert that is placed in the interior of the originally hollow tube made with one channel, and brazed fluid-tight at the points where it contacts the inside wall of the tube. In this manner, flow channels located side by side are produced with a triangular cross-section matching the cross-section of the corrugated insert. The flat tube is designed for use with R12 coolant and similar coolants, in which maximum operating pressures of approximately 20 bars typically occur. In one typical example, the tube wall thickness is 0.381 mm and the extent of the tube in the vertical direction is 1.91 mm. U.S. Pat. No. 5,372,188 teaches, as an alternative to installing a corrugated insert, the manufacture of flat tubes, with a row of channels with triangular cross-sections, from an extruded profile.
In offenlegungsschrift DE 38 43 305 A1 and U.S. Pat. Nos. 3,416,605, and 5,036,909, flat tubes with a plurality of flow channels located side by side with rectangular cross-sections are disclosed, with projections to increase the surface area possibly being provided on the edges of the channels.
An application that has recently gained in significance is heat exchangers for air conditioners, especially in motor vehicles that use coolant R-744, i.e. carbon dioxide. In this case, heat exchanger structures made of rectilinear or serpentine flat tubes are required which reliably withstand operating pressures above 100 bars. Although extruded multichannel flat tubes with flow channels having circular cross-sections and conventional flat non-profiled broad-sided tube outer surfaces have been considered for this application, it turns out that the heat exchange efficiency of these flat tubes requires improvement because of the relatively small heat-transmitting surfaces. The tubes are also relatively heavy for a given heat exchange efficiency.
The technical problem to be solved by the invention is to provide a multichannel flat tube of the type recited at the outset that is suitable for high-pressure applications with operating pressures of more than 100 bars, which is relatively light, and which provides relatively high heat exchange efficiency with a slight pressure drop in the coolant.
One disclosed manner by which this problem is solved is by providing a particularly constructed multichannel flat tube with flow channels with oval cross-sections and/or a corrugated external tube contour that matches the flow channels so that the flat tube is thinner in the vertical direction of the tube between each pair of flow channels than in the area of each flow channel. It turns out that the flat tube can be designed in this way so that, firstly, a relatively high heat exchange surface is available and, secondly, the remaining wall thicknesses are sufficient to provide the bursting strength required of the tube. The multichannel flat tube thus designed exhibits satisfactory heat exchange efficiency in terms of both volume and weight, with a relatively slight pressure drop in the coolant and a low weight. In particular, the multichannel that flat tube can be used for evaporators in condensers or gas coolers as well as internal heat exchangers in CO2 vehicle air conditioning systems. The multichannel flat tube can preferably be manufactured as an extruded flat tube, with the desired oval cross-sectional shape of the individual channels being produced by an extrusion process using suitably shaped dies.
According to one feature, the flow channels are oriented in a specific fashion, depending on the application, so that their major semiaxes lie perpendicularly or parallel to the transverse axis of the tube or are inclined at a specific acute angle relative to this axis.
According to another feature, the multichannel flat tube is designed so that the ratio of the major to the minor semiaxes of its oval flow channels lies between the values of 1 and 2, the ratio of the material cross-sectional area to the cross-sectional area through which flow occurs is between 1.4 and 4.5, in the case of the minor semiaxis of the respective flow channel that lies parallel to the transverse axis of the tube, the ratio of twice the value of the minor semiaxis to the period length of the flow channel row is between 0.4 and 0.9, and/or the ratio of twice the value of the major semiaxis to the flat tube thickness is between 0.4 and 0.8. These value ranges are especially favorable for achieving a high heat exchange efficiency on the one hand and a high bursting strength with the lowest possible weight on the other.
According to yet another feature, the channel edges in the circumferential direction have a path that is smooth and in the shape of an arc or a path that is corrugated or polygonal, the oval cross-section formed by a polygon with at least five, and preferably many more than five, corners. The corrugated or polygonal irregular surface contour, depending on the application, can have advantages from a manufacturing standpoint, especially advantages relating to flow behavior and heat exchange capability in addition to resistance to pressure.
In one preferred embodiment of the invention, a multichannel flat tube for a heat exchanger has a plurality of parallel flow channels aligned in a row, side by side, along a transverse axis of the tube. At least the inner flow channels have oval cross-sections, and major semiaxes of the inner flow channels are inclined relative to the transverse axis of the tube at an acute angle. The acute angle is preferably about 45xc2x0, and each outermost flow channel of the plurality of flow channels preferably has a circular cross-section.
Most preferably, in this particularly embodiment, a ratio of the major semiaxes lengths to the minor semiaxes lengths of the flow channels which have oval cross-sections is between 1.2 and 1.4, a ratio of material cross-sectional areas to cross-sectional areas of the flat tube through which flow can occur is between 1.0 and 4.0, a ratio of twice the value of each minor semiaxis length to a periodicity length of the row of flow channels is between 0.5 and 0.7, and a ratio of twice the value of each major semiaxis length to the tube thickness is between 0.6 and 0.8. Oval cross-sectional edges of the flow channels can be corrugated or can be formed of individual linear sections that form a polygon with at least five corners.